The present invention relates to a mask for measuring an optical aberration of an optical system such as a projection exposure system and a method of measuring the optical aberration, and more particularly to a mask for measuring even function aberrations such as a spherical aberration, an astigmation aberration and an image plane distortion, and a method of measuring the even function aberrations by use of the mask.
It has been known as a method of measuring the aberration of the optical system such as optical lenses that focus position over an image plane is measured to obtain a best focus position, so that a calculation is made on the basis of an inter-relationship between the obtained best focus position and positions over the image plane. This method will hereinafter be referred to as a bast focus method. This bast focus method is conducted as follows. An optical system is an object to be measured in aberration. This target optical system is used for focusing a measurement pattern onto a pattern image on the image plane, wherein plural transfer pattern images are formed on plural image planes which are slightly or finely different in focus position over the image planes in a direction parallel to an optical axis of the optical system. The obtained pattern images on the plural image planes are then observed by a microscope such as a high magnification optical microscope or an electron microscope in order to the image plane which provides the pattern image in the best focus state. The position in the direction parallel to the optical axis of the detected image plane is measured as the best focus position. This measured best focus position is then used for operations to enable measurements to the even function aberrations such as the spherical aberration, the astigmation aberration and the image plane distortion. In accordance with the best focus method, the detection to the best focus position of the pattern image over the image plane is made by that an operator observes the pattern image through the high magnification optical microscope or the electron microscope and recognizes the pattern image, which is lowest in deformation and size-variation thereof, to be the best focus. In case of the independent test patterns, the best focus is defined to be the transferred pattern image having a size variation within a predetermined range or within a half value of the range for recognizing that a reduction of a photo-sensitive resist film formed with the transferred pattern image is not remarkable. In case of a straight line-and-space test pattern, the best focus is defined to be the transferred pattern image having straight line patterns separated from each other in addition to the above image patterns described in the above case of the independent test patterns.
The above conventional method has the following disadvantages. The operator is required to obverse all of the pattern images and compare the pattern images to each other to decide the best focus. This means it difficult to realize an automation of the measurement. This means the above conventional method is the time-consuming procedure. The accuracy in measurement depends upon the individual skills of the operators, Namely, it is difficult to realize a highly accurate measurement.
A second conventional technique for measuring the aberration of the optical system is disclosed in Japanese laid-open patent publication No. 11-237310. It is necessary to form measuring mask patterns aligned on the basis of a period or a cycle which is defined by a predetermined operational expression. Actually, however, it is difficult to form such the measuring mask patterns. A space width between the adjacent measuring mask patterns is measured by the electron microscope for the purpose of measuring the just focus position of the pattern image. The need to use the electron microscope raises the above problem.
A third conventional technique for measuring the aberration of the optical system is also disclosed in Japanese laid-open patent publication No. 10-232183, An exposure of the straight line-and-space pattern having the constant straight line-width is made to obtain the pattern image so that the aberration is measured on the basis of the difference or variation in width of the straight lines of the pattern images. This technique is to measure the comma aberration, for example, the odd function aberration but never to measure the even function aberration.
In the above circumstances, it had been required to develop a novel mask for measuring the even function aberration of the optical system and the novel measuring method free from the above problem.
Accordingly, it is an object of the present invention to provide a novel mask for measuring the even function aberration of the optical system free from the above problems.
It is a further object of the present invention to provide a novel mask for measuring the even function aberration of the optical system, wherein the mask allows a realization of an automatic measurement.
It is a still further object of the present invention to provide a novel mask for measuring the even function aberration of the optical system, wherein the mask allows a realization of measurement to the best focus position in a short time and at a high accuracy.
It is yet a further object of the present invention to provide a novel method for measuring the even function aberration of the optical system free from the above problems.
It is a further object of the present invention to provide a novel method for measuring the even function aberration of the optical system, wherein the mask allows a realization of an automatic measurement.
It is a still further object of the present invention to provide a novel method for measuring the even function aberration of the optical system, wherein the mask allows a realization of measurement to the best focus position in a short time and at a high accuracy.
The present invention provides a mask for measuring an optical aberration, the mask including at least a measuring pattern comprising plural pattern parts being separated from each other, wherein the plural pattern parts provide individual widths which are simply increased on first and second directional axes non-parallel to each other and vertical to a plane of the mask, provided that the width of each of the plural pattern parts is unchanged at least on the first and second directional axes.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.